My many hemlocks have woolly adelgid. A horticulturalist with a tree service...

Asked April 24, 2014, 1:08 AM EDT

My many hemlocks have woolly adelgid. A horticulturalist with a tree service is suggesting that I have the trees sprayed with horticultural oil twice this year: once mid-May to mid-June and a second time mid-August to mid-June. I have two questions: 1. Is this pesticide agent and timing an effective treatment against woolly adelgid with minimal negative impact on native insects? 2. Is one treatment in late spring adequate or does the second treatment in late summer make enough of a difference to merit the extra expense? Thank you for your help.

Baltimore County Maryland

1 Response

The best way to treat for hemlock woolly adelgid would be applying a systemic soil drench that contains the active ingredient imidicloprid. Follow label directions for application. The hemlock does not flower and this product would have a very low impact on beneficials. Also, this insecticide offers control for about five years. Monitor the tree for signs of the insect before treating in the future.

If you do not want to use the systemic, see our website for control of the adelgid. You can use a dormant oil next season and spray with a summer rate of horticultural oil after all the eggs have hatched around mid-June. http://extension.umd.edu/learn/hemlock-woolly-adelgid-hg2
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